How to use existing UV unwrap texture on model
If the masks on your textures based off of a texture map that was made from a UV map are different it will not look to good. If you are tiling textures and say you want a different normal under a stone albedo(diffuse) megascan texture, it would work fine, it would just bring a different shape out of the albedo map. However, from the sounds of it, with a character as you are saying, when dealing with UV's the uv mask on the texture maps created from the UV's must match character UV's or it will not look very good.